1. Field of the Invention
The present invention relates generally to improvements in controlled directional drilling systems and more particularly pertains to a new and improved system and method for controlling the directional drilling of the borehole in a manner which will allow the borehole to be drilled in conformance with the proposed well plan.
2. Description of the Prior Art
In the field of controlled directional drilling of boreholes, until very recently, it had been the practice to use two separate down-hole systems. One system was used for drilling straight holes. A completely different system was used for causing the borehole to turn direction. The use of these two systems required that the entire drillstring be tripped or pulled from the borehole so that the down-hole system could be changed each time a change of direction was required. This type of system is described in SPE Paper No. 9649 entitled "Kicking Off in Large Diameter Holes" presented at the 1981 SPE Middle East Oil Technical Conference held in Manama, Bahrain, Mar. 9-12.
Although such systems produced results, they were unsatisfactory in several major respects. Considerable time was wasted as a result of the non-drilling activity occasioned by having to trip the bottom-hole assembly, either to follow the curvature in the well plan or to make corrections for unforeseen deviations in the borehole. This type of operation considerably increased the drilling time and decreased the rate of penetration (ROP). Another problem with this system is that the standard straight-hole drilling, bottom-hole assemblies utilized, deviated, sometimes considerably, from the well plan. In such a case, not only will the driller not reach his target but he will end up with a crooked hole, worn casing, stuck pipe, and expensive fishing jobs.
Deviating boreholes have been a subject of concern to this industry for a long time. Many approaches have been tried to first understand the multifacet problem and then to come up with a workable solution. One example of such an approach can be found in SPE Article No. 5070 entitled "Factors Affecting the Control of Borehole Angle in Straight and Directional Wells" presented at the SPE-AIME 49th Annual Fall Meeting in Houston, Tex., Oct. 6-9, 1974.
One of the problems confronting the industry with respect to the drilling of directional boreholes and deviation of boreholes from a well plan was obtaining arcurate information about the direction of the borehole. As a result, the industry developed systems for monitoring while drilling (MWD systems). In order to be able to ascertain when a borehole is deviating from its well plan and to be able to ascertain and control the direction of the borehole in order to follow the well plan, many different types of monitoring systems were developed.
A certain number of these systems are discussed, for example, in SPE paper No. 9224, entitled "A Case Study Comparison of Wells Drilled With and Without MWD Directional Surveys on the Claymore Platform in the North Sea" first presented at the SPE 55th Annual Technical Conference and Exhibition in Dallas on Sept. 21-24, 1980. Another example can be found in SPE paper No. 10053 entitled "Mud Pulse MWD Systems Report" first presented at the SPE 56th Oct. 4-7, 1981.
Another aspect of the problem confronting anyone attempting to drill a directional well at increased ROP without increasing cost, was the requirement of a powerful down-hole motor to turn the borehole, when required by the well plan, or to bring a deviated borehole back to the well plan. Such motors have been the focus of industry attention for considerable time and are now, to a reasonable extent, available. A description of a type of down-hole motor available at the present time can be found in SPE paper No. 13026 entitled "PDM vs. Turbodrill: A Drilling Comparison", presented at the 59th Annual Technical Conference and Exhibition in Houston, Tex. Sept. 16-19, 1984.
Even with all of these available pieces of a bottom hole assembly, the procedure for drilling a directional well was still to trip the drill string when a change in the direction of the borehole was called for by the well plan. In spite of the ability to monitor while drilling the direction of the borehole, the use of powerful downhole motors, and the ability to modify various factors like bit weight while the bottom-hole assembly was in the hole, boreholes still deviated from the well plan, requiring tripping of the drillstring and adjustment of the bottom-hole assemblies, as well as adjustment of the stabilizers on the drillstring. A method was needed which would considerably reduce, if not eliminate entirely, the round trips required with kick-off techniques and assembly changes for directional control of the borehole. A technique which shows promise and is currently being utilized by various operators in the industry is described in a paper by A. W. Kamp and R. Feenstra entitled "A Technique for Continuously Controlled Directional Drilling" presented at the Drilling Technology Conference of the International Association of Drilling Contractors in Dallas, Tex. on Mar. 19-21, 1984. The technique involves the use of a powerful down-hole motor and various ways of creating a side force on the bit or tilting the axis of the bit with respect to the axis of the borehole. It has been found that the bit will drill straight when both the drill string and the motor are rotated, and the bit will deviate in a desired direction when only the motor is rotated and the drillstring is kept stationary in a controlled tool face direction.
Since the introduction of this technique, various entities in this industry have developed a variety of bottom-hole assemblies to take advantage of its possibilities. Some systems have proven more promising than others. Each system in its own way, is searching for a reduction in drilling time by increasing rate of penetration and thereby reducing the cost of the well. One such system is described in an article entitled "Drill Faster, More Accurately With New Navagation System" published in World Oil on Aug. 1, 1985 and authored by T. Brassfield and H. Karlson.
The present invention is an improvement over the systems presently available and being tried by the industry to increase ROP of a directional well. The improved performance of the present invention is based on the fact that an overall system approach to each drilling job is utilized. In other words, the bottom-hole assembly is uniquely tailored for each proposed well plan by taking into consideration a myriad of facts such as hole condition, pump data, type of mud being utilized, type of formation being drilled, drilling assembly components, drilling flow rate, well plan, i.e. direction of the borehole after deviating from vertical, in addition to information about the drilling bit which includes bit size, bit type, bit pressure drop, and gauge length, as well as degrees of offset of the center line of the bit face from the center line of the borehole. This information is utilized according to the present invention to come up with a bottom hole assembly and method of building a bottom hole assembly which provides an ROP for directional wells which is considerably higher than was heretofore possible.